Patterns on islands

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Patterns on islands
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• Islanda relatively small area of suitable
  habitat isolated from a much larger area
  (source) of suitable, occupied habitat. For
  example, the continent nearest to an island would
  be considered the source.

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Observation

• Large islands have more species than smaller
  islands. A general rule is that as the land area
  increases 10 times, the number of species
  doubles.
• See page 429, textbook

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Lesser Antilles bird species
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S cAz

• S number of species
• c constant measuring the number of species per
  unit area. Insects, for example, will have a
  higher c than amphibians because there will be
  more insects per unit area than amphibians.
• A area of the island
• z constant measuring the slope of the line
  relating S and A. z is dependent on the type of
  organism and the island group and how distant
  island is from mainland

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S cAz

• z usually equals somewhere between .15 and .35.
  More poorly dispersing animals have higher zs, in
  other words, as island size increases, poorly
  dispersing animals show greater responses to the
  increase in size than animals that disperse well.
  
• zs are lower for terrestrial islands

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Smammal 1.188A0.326, Sbird 2.536A0.165, Brown
(1978)
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Different z values. Area effect is smaller on
mainland.
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Island biogeography theory

• Relatively successful ecological model that
  predicts the influence of immigration and
  extinction on the equilibrium number of species
  that will inhabit the island.

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Equilibrium

• Equilibrium number is a number we expect to be
  relatively constant over time, balanced by
  species immigrating to an island and other
  species going extinct.
• Dynamic equilibrium-refers to the fact that,
  although the number of species will be relatively
  constant, the species themselves are changing
  (because of immigration and extinction).

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• MacArthur and Wilson reasoned that the
  equilibrium species number will be influenced by
  both immigration to islands and extinction on
  islands, which will be influenced by distance of
  the island to the mainland and island size,
  respectively.

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• Turnover rate (on y-axis)the rate at which the
  identities of the species on the island changeis
  the point at which the immigration and extinction
  rates are equal

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• Near islands have higher immigration rates
  because likelihood of reaching a near island
  compared to a far island is greater
• Large islands have lower extinction rates because
  populations of any given species will be higher
  on large islands compared to small islands.
  Larger populations have a lower risk of
  extinction.

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Wilson and Simberloff test of model in Florida
Keys

• First, they censused four small islands covered
  with red mangrove (15 m across) and at different
  distances from the mainland. The censuses of
  insects and arthropods revealed what they
  expectedthe most species on the nearest island
  and the least on the farthest island.

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Wilson and Simberloff test of model in Florida
Keys

• Second, they hired a pest company to defaunate
  islands by covering them with rubber tents and
  using methyl bromide gas to kill the insects and
  arthropods

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Then they visited the islands periodically
afterwards to census the islands and determine
which species were there

• Support for equilibria idea--In less than a year,
  the nearest island had 44 species, where
  originally it had had 43. Furthest island had
  22, originally 25. Similar patterns on the other
  two islands. Numbers remained the same after two
  years.
• Support for the dynamic equilibrium ideaspecies
  identities changes while numbers stayed
  relatively constant They also discovered
  differences based on species differences.

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E1 is most isolated island (Simberloff Wilson
1970, Ecology 51934-937)
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They also discovered differences based on species
differences

• Spiders arrived quickly because of their
  ballooning habits but tended to go extinct
  relatively quickly
• Mites, blown in with dust, arrived more slowly
  but stayed longer
• Cockroaches, moths, and ants recolonized islands
  relatively quickly and persisted
• Centipedes and millipedes never recolonized over
  the two years of the experiment.

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• The researchers found higher immigration rates on
  the close islands, as expected
• Highest turnover rates were on close islands
• The size of the islands did not vary so they
  could not test the hypothesized relationship
  between island size and extinction rate

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More recent modifications to model

• Size of the island, as well as distance from the
  mainland, should affect immigration rates (target
  area effect)
• Distance to the mainland, as well as size of the
  island should affect extinction rates because of
  the rescue effect
• Evolution and interspecific interactions will
  mold island biotas
• Different taxonomic groups will reach equilibria
  at different points in time
• Small island effect

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Target Area Effect greater immigration rate on
larger islands
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• Rescue effectsmall populations of a species are
  rescued from extinction by the arrival of new
  immigrants of the same species

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Rescue effect (particularly on continental
islands) reduced turnover due to replacement
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Small Island Effect no area-diversity effect on small islands too few habitats                                                                            Niering, W.A. 1963. Terrestrial ecology of Kapingamarangi Atoll, Caroline Islands. Ecological Monographs 33131-160.    

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• Species richness of well-dispersing taxonomic
  groups (wind-dispersed plants, birds) appears to
  have reached equilibrium on Krakatoa but the
  richness of more poorly-dispersing taxonomic
  groups (animal-dispersed plants, non-flying
  mammals) has not.

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Krakatau Islands Biogeography Differential
immigration rates for plants with different
dispersal mechanisms
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Island patterns

• Insular refers to island
• Ecological release expansion of a species niche
  in the absence of competitors
• Harmonic insular biotas proportions of different
  types of organisms are similar on island and
  source
• Disharmonic insular biotas proportions of
  different types of organisms are different on
  island and source

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Patterns regarding three processes on islands

• Immigration
• Establishment
• Extinction

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Immigration

• Bats are well-represented on oceanic islands
  while many nonvolant mammals are not
• Bats colonized New Zealand and the Hawaiian
  islands while these areas have no other native
  mammals

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• Birds and the plants they eat are
  well-represented on oceanic islands, as are bird
  parasites

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• Amphibians and freshwater fish are poorly
  distributed on oceanic islands (New Zealand has
  no native freshwater fish)
• Rana cancrivora (crab-eating frog) and Bufo
  marina (marine or giant toad) have high
  tolerances for salt water both as tadpoles and
  adults and so are found on oceanic islands much
  more frequently than other amphibians.

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• Slugs are very intolerant of salt water and so
  are infrequently found on oceanic islands while
  land snails, which often thrive in dry habitats,
  are frequently found on such islands land
  snails are able to raft to islands

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• Large species, and those that stay active
  year-round are more likely to be found on islands
  (not necessarily distant oceanic islands). These
  types of species can use ice for travel.

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• Islands that are large and in archipelagos may be
  more likely to be found by dispersers, or islands
  that are in the route of particularly strong wind
  or water currents

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Establishment

• Species that are generalists are more likely to
  become established on islands than specialists
  (for ex. dung beetle generalists tend to have
  more successful introductions than specialists).

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• A study with land snails found that species with
  individuals that could self-fertilize were more
  likely to become established that species that
  could not do so

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• Individuals with high fecundity rates, i.e. large
  clutch or brood sizes, will likely become
  established more readily than other types of
  species

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• Islands that are large with a diversity of
  habitats and resources may be more hospitable to
  populations for establishment

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Extinction

• Large animals, carnivores, and specialists are
  more likely to become extinct on islands than
  small generalist herbivores. Smaller generalists
  will have larger population sizes than larger
  specialists and with larger population sizes
  there is less probability of extinction

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Evolutionary patterns on islands

• Reduced dispersal ability--so, ironically, the
  ancestors who dispersed well have descendants who
  don't disperse well
• Changes in body size

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Reduced dispersal ability

• Flightless birds and insects are common on
  oceanic islands
• Flightlessness has evolved in at least 8 orders
  of birds ostriches, ducks and geese, parrots,
  owls, doves, rails, storks and herons, and
  passerines
• New Zealand--25-35 of land and freshwater birds
  are (or were) flightless, 24 of Hawaii's endemic
  bird species

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Evolutionary scenario to account for
flightlessness?

• First of all, why do most birds fly?
• Predation is probably very important
• Those individuals who invested less in costly
  flight muscles would have more energy for other
  activities (like producing young) and would not
  suffer the losses from predation important on the
  mainland because many islands lack their
  traditional predators.

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• Flightlessness has evolved repeatedly in insects
  beetles, butterflies and moths, flies, ants bees
  and wasps, grasshoppers and crickets, true bugs
• On Madeira Island--off coast of Africa and
  Portugal--200 of 550 beetle species are
  flightless
• Insects also tend to be wingless at higher
  latitudes and in mountains

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Hypotheses to explain these patterns?

• Energy conservation
• Advantages to individuals of site fidelity
  (staying near their natal site)

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• Reduced dispersal ability is also evident in land
  snails and plants found on islands

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Changes in body size

• Woolly mammoth range shrunk from much of the
  northern Palearctic 20,000 ya to only Wrangel
  Island 10,000 years ago
• Size of woolly mammoths also shrunk from 6 tons
  to 2 tons by 2,000 years ago
• Size change must be positive for individuals to
  evolve but then may have positive consequences
  for the population

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• Individuals of small species tend to get larger
  on islands and individuals of larger species tend
  to get smallerwhy?

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Advantages of large size

• Larger individuals within a species can use more
  types of resources
• Larger individuals can produce more offspring
• Larger individuals tend to win in intraspecific
  competition
• Larger individuals have more stored energy to
  make it through times of food shortage

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Investigators suggest that these advantages will
be more important for individuals of small
species rather than individuals of large species

• Even a small elephant can use a variety of
  resources while a large rat may have a
  significant advantage over small rats in the
  variety of resources it can use
• Individuals of small species may show ecological
  release because of lack of competitors on
  islands.
• Advantages of small size to escape predation may
  be less useful in absence of typical predators

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Advantages of small size

• Smaller individuals can get by with less food and
  other resources
• Smaller individuals often use food more
  efficiently than larger individuals (assimilate
  energy from food)
• Smaller individuals can use smaller shelters and
  hiding spots than larger individuals

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Investigators suggest that these advantages will
be more important for individuals of large
species rather than individuals of small species

• A small elephant will be able to get by on much
  less of a resource base than a large elephant and
  resources for elephant-sized animals are more
  likely to be limited on islands
• The resources necessary to rats may not be as
  limited and so there may be no selective
  advantage to small rats vs. large rats.

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Island body size may also be a by-product of the
characteristics of immigrators

• Successful active dispersers may tend to be the
  larger individuals of a species (because, for
  example, larger individuals would have more
  energy reserves and hence be more likely to
  survive the journey to an oceanic island)

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Island body size may also be a by-product of the
characteristics of immigrators

• Successful passive dispersers may tend to be the
  smaller individuals of a species (smaller
  individuals are more likely to make it by being
  pushed by wind or water).

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• These patterns of successful immigrants may not
  be maintained in the island populations over time
  if there are counter- selective pressures for
  reasons discussed above and/or if immigration to
  the island is rare

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• In short, ecological release drives small animals
  to become bigger while resource limitation drives
  big animals to become smaller
• These patterns may not hold if the immigrant size
  patterns we just discussed are more influential
  than ecological release and resource
  limitationfor example when immigration is very
  common

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Birds and reptiles show some similar patterns to
mammals with many exceptions

• Exceptions may result because there is much more
  data for these taxonomic groups
• It is possible that birds and reptiles are under
  different selective pressures than mammals on
  islands.
• Reptiles are ectothermic and so resource
  limitation may not be a problem
• The lack of many medium-sized and large mammals
  on many islands may have allowed birds and
  reptiles to show ecological release

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Homo floresiensis discovered on Flores Island,
Indonesia, 2003
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The skull of Homo floresiensis can only hold a
brain that's about 380 cubic centimeters in size.
The modern human skull, at right, holds a brain
that measures between 1,400 and 1,500 cubic
centimeters. (Peter Brown)
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Homo floresiensis possible history

• Arrived on island as Homo erectus (first
  large-brained hominid from Africa and Asia)
• Unclear how they arrivedboats, swimming, and
  rafting all seem unlikely

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• Homo erectus probably averaged 510
• After arrival on the island, the smallest
  individuals may have survived best because of
  resource limitation (Flores island is only 31 sq.
  miles in area)

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• Hot, humid weather of the region may also have
  favored small individuals, who, with a greater
  surface area to volume would have been able to
  cool off faster and would have generated less
  heat when they moved
• Appears the individuals lived in caves

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• Remains of female individual found with remains
  of miniature Stegodon sp., Komodo dragon, and
  burned bones of birds, rats, and fish, and stones
  tools, in cave.

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• Since publication describing the new species was
  submitted to Nature, authors have found remains
  of more individuals that appear to be 95,000 to
  13,000 years old
• Some other anthopologists are skeptical of
  calling it a new species

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• The new species may have been killed off when a
  volcano erupted on the island 12,000 years ago
• Signs of modern humans on the island are 11,000
  years old

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Very controversial today

• Oct 2005, Nature published descriptions of bones
  of 9 individuals of Homo floriensis, some
  thousands of years apart in agescientists argue
  that the number of specimens and the time span,
  refute the idea that they simply discovered
  abnormal individuals.
• Several studies argue that the skull is probably
  from a small-bodied modern human who had a
  genetic condition, microcephaly. Individuals
  with microcephaly have small heads.

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2007 papers

• Homo floresiensis were not microencepahlic (based
  on comparisons with truly microencephali
  individuals, Falk et al. 2007)
• Hand bones of H. floresiensis more similar to
  chimps or early hominids than to modern humans
  (Tocheri et al. 2007)
• Falk, D et al, (2007). "Brain shape in human
  microcephalics and Homo floresiensis".
  Proceedings of the National Academy of Sciences
  104 (7) 2513. doi10.1073/pnas.0609185104. PMID
  17277082. Retrieved on 2008-03-05. "lay summary"
  (2007-01-29). Retrieved on 2008-03-05.
• Tocheri et al, (2007). "The Primitive Wrist of
  Homo floresiensis and Its Implications for
  Hominin Evolution". Science 317 (5845) 1743.
  doi10.1126/science.1147143. PMID 17885135. "lay
  summary" (2007-09-20). Retrieved on 2008-03-05.

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As of 2009, debate continues

• Weston and Lister 2009 suggest that small brain
  size of H. floresiensis may be consistent with
  brain size evolution of other mammalian groups on
  islands.